Sapropterin hydrochloride (L-tetrahydrobiopterin dihydrochloride) is widely used as a therapeutic agent for atypical hyperphenylalaninemia. Sapropterin hydrochloride is produced by first producing a hydrazone derivative from L-rhamnose through L-rhamnose diethyl mercaptal (REM) and 5-deoxy-L-arabinose (5-DA), then acetylating the resultant hydrazone derivative and reacting the acetylated hydrazone derivative with 6-hydroxy-2,4,5-triaminopyrimidine, eliminating acetyl groups therefrom, and subsequently subjecting the thus deacetylated product to asymmetric reduction (cf. patent literature 1 and nonpatent literature 1).

As described above, the hydrazone derivative is an important intermediate in the production process of sapropterin hydrochloride. However, this method has so far been disadvantageously affected by the difficulties that the steps covering from the L-rhamnose diethyl mercaptal (REM) to 5-DA above in the production process of hydrazone derivative involve a very low reaction yield and a very inferior reproducibility along with a long reaction time and complicated postprocessing.
[Patent literature 1] Japanese published unexamined patent application JP A S59-186986
[Nonpatent literature 1] Helv. Chim. Acta 68(6)1639-1643 (1985)
[Patent literature 2] U.S. Pat. No. 3,505,329
[Nonpatent literature 2] J. Am. Chem. Soc. 96. 6781 (1974)